I have a 1968 455 two-barrel engine with C Heads (I assume small valves).
Are these small C valves the same diameter as those on the 1971 4-barrel G heads? Which is the better of the two?

 

There's not really much difference between the C and G heads as far as flow and combustion chamber volume (assuming the same size valves). I doubt you could detect any difference in performance between the two.

 

i have had c heads and edelbrocks. the edelbrocks are c heads improved the late great rip joe mondello with edelbrock made the aluminums from scratch and improved the weak areas or just really blueprinted a new design. 2 c heads by themselves are just about the best performers. but need hardened seats installed. the vaunted f heads are supposed to be the best but the seats where supposedly weak and fell out. the d heads were used on hurst olds cars from what i know but i cannot say for sure if they flow among the best the hurst olds is a legend from the late sixties and very expensive when i grew up and rarely seen and never run due to its collector status. the k heads where used on 71 442 cars and although they had hardened seats i have also wondered if they were designed for low compression uses since these cars used low lead gas. the f ,aluminums and c heads seem to be the best flowers from all sources known of.

 

Olds only used two different valve sizes on BBO heads. Small valve heads used 2.000" intakes and 1.625" exhausts. Big valve heads used 2.070" intakes and 1.625" exhausts. Apparently some of the Ga heads from 1972 used 1.685" exhaust valves, but this slightly larger diameter might have more to do with the unusual 30 degree face angle rather than any effective flow increase.

As for port flow, there is a lot of BS on the web and a lot of urban myths. The reality is that except for the smogger J heads, all BBO heads probably flow within 5% or so of each other in the as-cast condition, assuming same-size valves. I find it especially hilarious when people claim the "C" heads are the best-flowing heads that Olds ever made. If that were true, why would Olds use crappier heads on the top performance motors? Did the W-30s make so much power that they had to be detuned with those "poor flowing" D and F heads?

 

I have a 71 442 numbers matching 455 motor. I couldn't be positive but I'm pretty sure the heads were never changed and they are G heads. I've owned the car 29 years and my friend I bought it off of had it for 10. A lot can happen in 7 years but they were the early years & there were no other mods to the car aside from a shotty drivers side hit repair. Where do the K heads come in?

 

G heads were correct for a 1971 motor. The K and KA heads were primarily used on non-automotive applications in the 1970s. Olds sold a LOT of 455s for marine use in that time and most came with K or KA heads. These did not have the restrictive smog motor ports of the J heads and superseded all the performance heads in the parts book by the mid-70s. Apparently these might have been installed on a handful of the performance applications in the mid-70s, like H/Os, but I haven't actually found any documentation to prove that.

 

According to a chart I saw the A head has the best intake port and the J has the worst exhaust port. The F head has a partially blocked exhaust crossover so only one port crosses over to the other side. And the divider between the two center exhaust port properly extends out to the flange unlike every other Olds head. I have a pair of articles on 455 Olds build ups by Joe Mondello. One was a ported C head 455 the other was an Edelbrock head 455. He achieved about the same 535 hp on both.

 

The problem with pretty much every assessment of port flow rate is that it is based on one or two tests. That's not a statistically valid sample size to account for manufacturing tolerances. The margin of error is enough that they are all pretty much equivalent in the as-cast state. If you plan to port them, it doesn't matter where you start. A head castings do have the advantage of not having A.I.R. bumps in the ports.